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It is increasingly recognised that chronic fatigue syndrome (CFS) is heterogeneous. A significant proportion of patients fulfilling operative criteria for a diagnosis of CFS will also fulfill criteria for a psychiatric disorder, such as depression or somatisation. Failure to recognise this heterogeneity prejudices attempts to understand CFS in cross sectional studies. In this issue (pp 302–307) Fulcheret al report a study of muscle strength, aerobic exercise capacity, and functional incapacity in a group of patients with CFS without concurrent psychiatric disorder, compared with patients with major depression and a group of normal but sedentary subjects.1 In an incremental treadmill exercise test, patients with CFS and depressed patients had lower peak oxygen consumption rates, maximal heart rates, and plasma lactate concentrations than the sedentary controls; but this reflected the shorter duration of exercise tolerated by these patients. At submaximal work rates, patients with CFS and depressed patients experienced greater perception of effort than sedentary controls at the same level of work. This is in keeping with the finding that such patients show greater sensitivity to bodily sensations than normal subjects. Overall, there was little difference between the patients with CFS and the depressed patients in exercise characteristics, yet the patients with CFS reported significantly greater degrees of physical fatigue and physical incapacity.
The authors did find one important difference, however. The patients with CFS were significantly weaker than either the depressed patients or the sedentary controls as judged by measurement of quadriceps strength. This is the first study to demonstrate such physical weakness in patients with CFS and the authors suggest that this is because they studied a more coherent group of patients than others have previously.
Fatigue is a complex symptom. In its strictest sense the word means “inability to sustain force”, indicating dysfunction in the neuraxis or neuromuscular apparatus, whether physiological (after strenuous exercise) or due to disease processes. In its colloquial context (fatigare-to tire), it is the sensation experienced when the effort to perform work, whether physical, mental or both, seems disproportionate for the task involved. Physiological fatigue recovers with rest. Chronic fatigue typically does not. Although chronic fatigue is a very common complication of a wide range of medical and neurological diseases, such as multiple sclerosis and Parkinson's disease, it can also occur in the absence of readily definable pathology, notably in CFS.
What is the cause of this physical weakness in CFS? The authors think that it is related to physical deconditioning due to inactivity and this view is supported by improvement in wellbeing, strength, and exercise capacity after a graded aerobic exercise programme.2 However, graded exercise therapy improves tolerance and exercise capacity in many disorders, including muscle diseases such as mitochondrial myopathies.3
Some patients with CFS show abnormal increases in plasma lactate after exercise at low work rates.4 Heart rate responses to exercise in that study did not suggest that these patients with CFS were more “deconditioned” than those with normal lactate responses. They also proved less likely to have psychiatric disorder. Furthermore, recent spectroscopic studies of the forearm muscles, which are not usually subject to the effects of deconditioning, showed abnormal muscle energy metabolism in such cases.5 Deconditioning may well be an important factor in the pathogenesis of CFS, but these findings raise the possibility that some patients with CFS have a form of metabolic myopathy.
Whatever the mechanisms underlying “fatigue”, exercise therapy is likely to become an increasingly important therapeutic modality in various fields and particularly in the management of chronic fatigue syndromes.